When drilling in rotary mode, with rotation of a drill string being used to rotate a drill bit, and with a positive displacement drilling motor in the drill string, the drill bit will generally rotate at a greater speed than the drill string. This is because the drilling motor rotates the drill bit, and the drill string above the drilling motor rotates the drilling motor.
Unfortunately, as weight on the bit increases, and/or as torque increases (e.g., due to encountering a harder subterranean formation, etc.), the rotational speed of the bit can decrease to a point where the drill string above the drilling motor rotates at a greater speed than the bit. This situation can cause damage to the drilling motor and/or other drilling equipment in the drill string.
Therefore, it will be appreciated that improvements are continually needed in the art of constructing and operating downhole drilling tools. Such improvements may be used in the situation discussed above, or in other drilling situations.